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Examples of rwtime-nd.stp, the Linux SystemTap version.
rwtime-nd.stp measures syscall read() and write() times, summarizing them
as histograms. A process name can be specified. For example, to measure
read() and write() latency from processes named "node":
# ./rwtime-nd.stp node
Tracing read/write syscalls for processes named "node"... Hit Ctrl-C to end.
^C
syscall read latency (ns):
value |-------------------------------------------------- count
512 | 0
1024 | 0
2048 | 2
4096 |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@ 278
8192 | 1
16384 | 5
32768 | 0
65536 | 0
syscall write latency (ns):
value |-------------------------------------------------- count
4096 | 0
8192 | 0
16384 |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@ 135
32768 |@@ 8
65536 | 0
131072 | 0
Those reads and writes were very fast, between 4 and 32 microseconds.
Here's an example with much higher latencies: processes named "bash":
# ./rwtime-nd.stp bash
Tracing read/write syscalls for processes named "bash"... Hit Ctrl-C to end.
^C
syscall read latency (ns):
value |-------------------------------------------------- count
256 | 0
512 | 0
1024 |@ 1
2048 |@ 1
4096 | 0
8192 | 0
16384 | 0
32768 |@ 1
65536 |@@@@@@ 6
131072 |@@@@@@@@@@@ 11
262144 | 0
524288 | 0
~
2097152 | 0
4194304 | 0
8388608 |@ 1
16777216 | 0
33554432 |@ 1
67108864 |@@@ 3
134217728 |@@@@@@@@@ 9
268435456 |@@@ 3
536870912 |@@ 2
1073741824 |@@ 2
2147483648 | 0
4294967296 |@ 1
8589934592 | 0
17179869184 | 0
syscall write latency (ns):
value |-------------------------------------------------- count
512 | 0
1024 | 0
2048 |@@@@@@@@@@@@@@@@@@@@@ 21
4096 |@@@@@@@@@@@@@@@@@@@@@@ 22
8192 |@@@@@@@@@@@@@@@@@@ 18
16384 | 0
32768 | 0
The high read time, with a mode between 134 and 268 milliseconds, corresponds
to inter-keystroke latency as the bash shell reads keystrokes.